Sardine and Watercress Salad with Shredded Potato Cake

1 tablespoon Dijon-style mustard , or to taste

1/4 cup olive oil

1 tablespoon water

a 1/2-pound russet (baking) potato

1 tablespoon unsalted butter

1 small bunch of watercress , rinsed, spun dry, and coarse stems discarded (about 3 cups)

1/2 red onion , sliced thin, separated into rings, and soaked in ice water to cover for 30 minutes

a 3 3/4-ounce can brisling sardines packed in oil, drained well